Abstract
A molecule, molecular aggregate, or protein that cannot be superimposed on its mirror image presents chirality. Most living systems are organized by chiral building blocks, such as amino acids, peptides, and carbohydrates, and any change in their molecular structure (i.e., handedness or helicity) alters the biochemical and pharmacological functions of the molecules, many of which take place at surfaces. Therefore, studying surface chirogenesis at the nanoscale is fundamentally important and derives various applications. For example, since proteins contain highly ordered secondary structures, the intrinsic chirality can be served as a signature to measure the dynamics of protein adsorption and protein conformational changes at biological surfaces. Furthermore, a better understanding of chiral recognition and separation at bio-nanointerfaces is helpful to standardize chiral drugs and monitor the synthesis of adsorbents with high precision. Thus, exploring the changes in surface chirality with polarized excitations would provide structural and biochemical information of the adsorbed molecules, which has led to the development of label-free and noninvasive measurement tools based on linear and nonlinear optical effects. In this review, the principles and selected applications of linear and nonlinear optical methods for quantifying surface chirality are introduced and compared, aiming to conceptualize new ideas to address critical issues in surface biochemistry.
Highlights
Chirality of molecular architectures has become a subject of increasing interest to the chemical, biological, and pharmaceutical science community, largely because of its immense importance in understanding the structure and function of biological systems (Green et al, 2016; Jiang et al, 2017b; Gogoi et al, 2019; Zhao et al, 2020)
Unlike the OA effects measured by linear optical methods that are mainly contributed from magnetic dipole or quadrupole interactions, C-second-harmonic generation (SHG) and C-sum-frequency generation (SFG) are electric dipole–allowed transitions (Petralli-Mallow et al, 1993; Byers et al, 1994a; Ji and Shen, 2006), manifesting that they are correlated with a unique molecular structure and a polarized excitation (Simpson, 2002)
In addition to the inverted circular dichroism (CD) spectra observed for right- and left-handed helices with sign reversal occurring near the plasmon resonance frequency of the Au nanoparticle, the study revealed that the amplitude of the CD signal got increased and red shifted with the increase of the diameter of the nanoparticles
Summary
Chirality of molecular architectures has become a subject of increasing interest to the chemical, biological, and pharmaceutical science community, largely because of its immense importance in understanding the structure and function of biological systems (Green et al, 2016; Jiang et al, 2017b; Gogoi et al, 2019; Zhao et al, 2020). Unlike the OA effects measured by linear optical methods that are mainly contributed from magnetic dipole or quadrupole interactions, C-SHG and C-SFG are electric dipole–allowed transitions (Petralli-Mallow et al, 1993; Byers et al, 1994a; Ji and Shen, 2006), manifesting that they are correlated with a unique molecular structure and a polarized excitation (Simpson, 2002). For this reason, the measurement techniques are designed with specific polarization settings and experimental geometry. We will deliberately concentrate on the applications of linear and nonlinear chiroptical methods, instead of the development of theory, in studying the chirality at liquid, solid, film, and nanoparticle (NP) surfaces, air/liquid, liquid/liquid, and solid/liquid interfaces, membranes, etc.
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